PostgreSQL's statistics collector is a subsystem that supports collection and reporting of information about server activity. Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. It can also count calls to user-defined functions and the total time spent in each one.

PostgreSQL also supports reporting dynamic information about exactly what is going on in the system right now, such as the exact command currently being executed by other server processes, and which other connections exist in the system. This facility is independent of the collector process.

Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. This is controlled by configuration parameters that are normally set in postgresql.conf. (See Chapter 18 for details about setting configuration parameters.)

The parameter track_activities enables monitoring of the current command being executed by any server process.

The parameter track_counts controls whether statistics are collected about table and index accesses.

The parameter track_functions enables tracking of usage of user-defined functions.

Normally these parameters are set in postgresql.conf so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the SET command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.)

The statistics collector transmits the collected information to other PostgreSQL processes through temporary files. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp by default. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. When the server shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. When recovery is performed at server start (e.g. after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset.

Several predefined views, listed in Table 27-1, are available to show the current state of the system. There are also several other views, listed in Table 27-2, available to show the results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section 27.2.3.

When using the statistics to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process transmits new statistical counts to the collector just before going idle; so a query or transaction still in progress does not affect the displayed totals. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500 ms unless altered while building the server). So the displayed information lags behind actual activity. However, current-query information collected by track_activities is always up-to-date.

Another important point is that when a server process is asked to display any of these statistics, it first fetches the most recent report emitted by the collector process and then continues to use this snapshot for all statistical views and functions until the end of its current transaction. So the statistics will show static information as long as you continue the current transaction. Similarly, information about the current queries of all sessions is collected when any such information is first requested within a transaction, and the same information will be displayed throughout the transaction. This is a feature, not a bug, because it allows you to perform several queries on the statistics and correlate the results without worrying that the numbers are changing underneath you. But if you want to see new results with each query, be sure to do the queries outside any transaction block. Alternatively, you can invoke pg_stat_clear_snapshot(), which will discard the current transaction's statistics snapshot (if any). The next use of statistical information will cause a new snapshot to be fetched.

A transaction can also see its own statistics (as yet untransmitted to the collector) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. These numbers do not act as stated above; instead they update continuously throughout the transaction.

Table 27-1. Dynamic Statistics Views

View Name

Description

pg_stat_activity

One row per server process, showing information related to the current activity of that process, such as state and current query. See pg_stat_activity for details.

pg_stat_replication

One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See pg_stat_replication for details.

pg_stat_ssl

One row per connection (regular and replication), showing information about SSL used on this connection. See pg_stat_ssl for details.

Table 27-2. Collected Statistics Views

View Name

Description

pg_stat_archiver

One row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details.

pg_stat_bgwriter

One row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details.

pg_stat_database

One row per database, showing database-wide statistics. See pg_stat_database for details.

pg_stat_database_conflicts

One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details.

pg_stat_all_tables

One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details.

pg_stat_sys_tables

Same as pg_stat_all_tables, except that only system tables are shown.

pg_stat_user_tables

Same as pg_stat_all_tables, except that only user tables are shown.

pg_stat_xact_all_tables

Similar to pg_stat_all_tables, but counts actions taken so far within the current transaction (which are not yet included in pg_stat_all_tables and related views). The columns for numbers of live and dead rows and vacuum and analyze actions are not present in this view.

pg_stat_xact_sys_tables

Same as pg_stat_xact_all_tables, except that only system tables are shown.

pg_stat_xact_user_tables

Same as pg_stat_xact_all_tables, except that only user tables are shown.

pg_stat_all_indexes

One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details.

pg_stat_sys_indexes

Same as pg_stat_all_indexes, except that only indexes on system tables are shown.

pg_stat_user_indexes

Same as pg_stat_all_indexes, except that only indexes on user tables are shown.

pg_statio_all_tables

One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details.

pg_statio_sys_tables

Same as pg_statio_all_tables, except that only system tables are shown.

pg_statio_user_tables

Same as pg_statio_all_tables, except that only user tables are shown.

pg_statio_all_indexes

One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details.

pg_statio_sys_indexes

Same as pg_statio_all_indexes, except that only indexes on system tables are shown.

pg_statio_user_indexes

Same as pg_statio_all_indexes, except that only indexes on user tables are shown.

pg_statio_all_sequences

One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details.

pg_statio_sys_sequences

Same as pg_statio_all_sequences, except that only system sequences are shown. (Presently, no system sequences are defined, so this view is always empty.)

pg_statio_user_sequences

Same as pg_statio_all_sequences, except that only user sequences are shown.

pg_stat_user_functions

One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details.

pg_stat_xact_user_functions

Similar to pg_stat_user_functions, but counts only calls during the current transaction (which are not yet included in pg_stat_user_functions).

The per-index statistics are particularly useful to determine which indexes are being used and how effective they are.

The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. When the number of actual disk reads is much smaller than the number of buffer hits, then the cache is satisfying most read requests without invoking a kernel call. However, these statistics do not give the entire story: due to the way in which PostgreSQL handles disk I/O, data that is not in the PostgreSQL buffer cache might still reside in the kernel's I/O cache, and might therefore still be fetched without requiring a physical read. Users interested in obtaining more detailed information on PostgreSQL I/O behavior are advised to use the PostgreSQL statistics collector in combination with operating system utilities that allow insight into the kernel's handling of I/O.

Table 27-3. pg_stat_activity View

Column

Type

Description

datid

oid

OID of the database this backend is connected to

datname

name

Name of the database this backend is connected to

pid

integer

Process ID of this backend

usesysid

oid

OID of the user logged into this backend

usename

name

Name of the user logged into this backend

application_name

text

Name of the application that is connected to this backend

client_addr

inet

IP address of the client connected to this backend. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum.

client_hostname

text

Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled.

client_port

integer

TCP port number that the client is using for communication with this backend, or -1 if a Unix socket is used

backend_start

timestamp with time zone

Time when this process was started, i.e., when the client connected to the server

xact_start

timestamp with time zone

Time when this process' current transaction was started, or null if no transaction is active. If the current query is the first of its transaction, this column is equal to the query_start column.

query_start

timestamp with time zone

Time when the currently active query was started, or if state is not active, when the last query was started

state_change

timestamp with time zone

Time when the state was last changed

waiting

boolean

True if this backend is currently waiting on a lock

state

text

Current overall state of this backend. Possible values are:

active: The backend is executing a query.

idle: The backend is waiting for a new client command.

idle in transaction: The backend is in a transaction, but is not currently executing a query.

idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error.

fastpath function call: The backend is executing a fast-path function.

disabled: This state is reported if track_activities is disabled in this backend.

backend_xid

xid

Top-level transaction identifier of this backend, if any.

backend_xmin

xid

The current backend's xmin horizon.

query

text

Text of this backend's most recent query. If state is active this field shows the currently executing query. In all other states, it shows the last query that was executed.

The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process.

Note: The waiting and state columns are independent. If a backend is in the active state, it may or may not be waiting. If the state is active and waiting is true, it means that a query is being executed, but is being blocked by a lock somewhere in the system.

Table 27-4. pg_stat_replication View

Column

Type

Description

pid

integer

Process ID of a WAL sender process

usesysid

oid

OID of the user logged into this WAL sender process

usename

name

Name of the user logged into this WAL sender process

application_name

text

Name of the application that is connected to this WAL sender

client_addr

inet

IP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine.

client_hostname

text

Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled.

client_port

integer

TCP port number that the client is using for communication with this WAL sender, or -1 if a Unix socket is used

backend_start

timestamp with time zone

Time when this process was started, i.e., when the client connected to this WAL sender

Last transaction log position replayed into the database on this standby server

sync_priority

integer

Priority of this standby server for being chosen as the synchronous standby

sync_state

text

Synchronous state of this standby server

The pg_stat_replication view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. Only directly connected standbys are listed; no information is available about downstream standby servers.

Table 27-5. pg_stat_ssl View

Column

Type

Description

pid

integer

Process ID of a backend or WAL sender process

ssl

boolean

True if SSL is used on this connection

version

text

Version of SSL in use, or NULL if SSL is not in use on this connection

cipher

text

Name of SSL cipher in use, or NULL if SSL is not in use on this connection

bits

integer

Number of bits in the encryption algorithm used, or NULL if SSL is not used on this connection

compression

boolean

True if SSL compression is in use, false if not, or NULL if SSL is not in use on this connection

clientdn

text

Distinguished Name (DN) field from the client certificate used, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated if the DN field is longer than NAMEDATALEN (64 characters in a standard build)

The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection.

Table 27-6. pg_stat_archiver View

Column

Type

Description

archived_count

bigint

Number of WAL files that have been successfully archived

last_archived_wal

text

Name of the last WAL file successfully archived

last_archived_time

timestamp with time zone

Time of the last successful archive operation

failed_count

bigint

Number of failed attempts for archiving WAL files

last_failed_wal

text

Name of the WAL file of the last failed archival operation

last_failed_time

timestamp with time zone

Time of the last failed archival operation

stats_reset

timestamp with time zone

Time at which these statistics were last reset

The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster.

Table 27-7. pg_stat_bgwriter View

Column

Type

Description

checkpoints_timed

bigint

Number of scheduled checkpoints that have been performed

checkpoints_req

bigint

Number of requested checkpoints that have been performed

checkpoint_write_time

double precision

Total amount of time that has been spent in the portion of checkpoint processing where files are written to disk, in milliseconds

checkpoint_sync_time

double precision

Total amount of time that has been spent in the portion of checkpoint processing where files are synchronized to disk, in milliseconds

buffers_checkpoint

bigint

Number of buffers written during checkpoints

buffers_clean

bigint

Number of buffers written by the background writer

maxwritten_clean

bigint

Number of times the background writer stopped a cleaning scan because it had written too many buffers

buffers_backend

bigint

Number of buffers written directly by a backend

buffers_backend_fsync

bigint

Number of times a backend had to execute its own fsync call (normally the background writer handles those even when the backend does its own write)

buffers_alloc

bigint

Number of buffers allocated

stats_reset

timestamp with time zone

Time at which these statistics were last reset

The pg_stat_bgwriter view will always have a single row, containing global data for the cluster.

Table 27-8. pg_stat_database View

Column

Type

Description

datid

oid

OID of a database

datname

name

Name of this database

numbackends

integer

Number of backends currently connected to this database. This is the only column in this view that returns a value reflecting current state; all other columns return the accumulated values since the last reset.

xact_commit

bigint

Number of transactions in this database that have been committed

xact_rollback

bigint

Number of transactions in this database that have been rolled back

blks_read

bigint

Number of disk blocks read in this database

blks_hit

bigint

Number of times disk blocks were found already in the buffer cache, so that a read was not necessary (this only includes hits in the PostgreSQL buffer cache, not the operating system's file system cache)

tup_returned

bigint

Number of rows returned by queries in this database

tup_fetched

bigint

Number of rows fetched by queries in this database

tup_inserted

bigint

Number of rows inserted by queries in this database

tup_updated

bigint

Number of rows updated by queries in this database

tup_deleted

bigint

Number of rows deleted by queries in this database

conflicts

bigint

Number of queries canceled due to conflicts with recovery in this database. (Conflicts occur only on standby servers; see pg_stat_database_conflicts for details.)

temp_files

bigint

Number of temporary files created by queries in this database. All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the log_temp_files setting.

temp_bytes

bigint

Total amount of data written to temporary files by queries in this database. All temporary files are counted, regardless of why the temporary file was created, and regardless of the log_temp_files setting.

deadlocks

bigint

Number of deadlocks detected in this database

blk_read_time

double precision

Time spent reading data file blocks by backends in this database, in milliseconds

blk_write_time

double precision

Time spent writing data file blocks by backends in this database, in milliseconds

stats_reset

timestamp with time zone

Time at which these statistics were last reset

The pg_stat_database view will contain one row for each database in the cluster, showing database-wide statistics.

Table 27-9. pg_stat_database_conflicts View

Column

Type

Description

datid

oid

OID of a database

datname

name

Name of this database

confl_tablespace

bigint

Number of queries in this database that have been canceled due to dropped tablespaces

confl_lock

bigint

Number of queries in this database that have been canceled due to lock timeouts

confl_snapshot

bigint

Number of queries in this database that have been canceled due to old snapshots

confl_bufferpin

bigint

Number of queries in this database that have been canceled due to pinned buffers

confl_deadlock

bigint

Number of queries in this database that have been canceled due to deadlocks

The pg_stat_database_conflicts view will contain one row per database, showing database-wide statistics about query cancels occurring due to conflicts with recovery on standby servers. This view will only contain information on standby servers, since conflicts do not occur on master servers.

Table 27-10. pg_stat_all_tables View

Column

Type

Description

relid

oid

OID of a table

schemaname

name

Name of the schema that this table is in

relname

name

Name of this table

seq_scan

bigint

Number of sequential scans initiated on this table

seq_tup_read

bigint

Number of live rows fetched by sequential scans

idx_scan

bigint

Number of index scans initiated on this table

idx_tup_fetch

bigint

Number of live rows fetched by index scans

n_tup_ins

bigint

Number of rows inserted

n_tup_upd

bigint

Number of rows updated (includes HOT updated rows)

n_tup_del

bigint

Number of rows deleted

n_tup_hot_upd

bigint

Number of rows HOT updated (i.e., with no separate index update required)

n_live_tup

bigint

Estimated number of live rows

n_dead_tup

bigint

Estimated number of dead rows

n_mod_since_analyze

bigint

Estimated number of rows modified since this table was last analyzed

last_vacuum

timestamp with time zone

Last time at which this table was manually vacuumed (not counting VACUUM FULL)

last_autovacuum

timestamp with time zone

Last time at which this table was vacuumed by the autovacuum daemon

last_analyze

timestamp with time zone

Last time at which this table was manually analyzed

last_autoanalyze

timestamp with time zone

Last time at which this table was analyzed by the autovacuum daemon

vacuum_count

bigint

Number of times this table has been manually vacuumed (not counting VACUUM FULL)

autovacuum_count

bigint

Number of times this table has been vacuumed by the autovacuum daemon

analyze_count

bigint

Number of times this table has been manually analyzed

autoanalyze_count

bigint

Number of times this table has been analyzed by the autovacuum daemon

The pg_stat_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about accesses to that specific table. The pg_stat_user_tables and pg_stat_sys_tables views contain the same information, but filtered to only show user and system tables respectively.

Table 27-11. pg_stat_all_indexes View

Column

Type

Description

relid

oid

OID of the table for this index

indexrelid

oid

OID of this index

schemaname

name

Name of the schema this index is in

relname

name

Name of the table for this index

indexrelname

name

Name of this index

idx_scan

bigint

Number of index scans initiated on this index

idx_tup_read

bigint

Number of index entries returned by scans on this index

idx_tup_fetch

bigint

Number of live table rows fetched by simple index scans using this index

The pg_stat_all_indexes view will contain one row for each index in the current database, showing statistics about accesses to that specific index. The pg_stat_user_indexes and pg_stat_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.

Indexes can be used by simple index scans, "bitmap" index scans, and the optimizer. In a bitmap scan the output of several indexes can be combined via AND or OR rules, so it is difficult to associate individual heap row fetches with specific indexes when a bitmap scan is used. Therefore, a bitmap scan increments the pg_stat_all_indexes.idx_tup_read count(s) for the index(es) it uses, and it increments the pg_stat_all_tables.idx_tup_fetch count for the table, but it does not affect pg_stat_all_indexes.idx_tup_fetch. The optimizer also accesses indexes to check for supplied constants whose values are outside the recorded range of the optimizer statistics because the optimizer statistics might be stale.

Note: The idx_tup_read and idx_tup_fetch counts can be different even without any use of bitmap scans, because idx_tup_read counts index entries retrieved from the index while idx_tup_fetch counts live rows fetched from the table. The latter will be less if any dead or not-yet-committed rows are fetched using the index, or if any heap fetches are avoided by means of an index-only scan.

Table 27-12. pg_statio_all_tables View

Column

Type

Description

relid

oid

OID of a table

schemaname

name

Name of the schema that this table is in

relname

name

Name of this table

heap_blks_read

bigint

Number of disk blocks read from this table

heap_blks_hit

bigint

Number of buffer hits in this table

idx_blks_read

bigint

Number of disk blocks read from all indexes on this table

idx_blks_hit

bigint

Number of buffer hits in all indexes on this table

toast_blks_read

bigint

Number of disk blocks read from this table's TOAST table (if any)

toast_blks_hit

bigint

Number of buffer hits in this table's TOAST table (if any)

tidx_blks_read

bigint

Number of disk blocks read from this table's TOAST table indexes (if any)

tidx_blks_hit

bigint

Number of buffer hits in this table's TOAST table indexes (if any)

The pg_statio_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about I/O on that specific table. The pg_statio_user_tables and pg_statio_sys_tables views contain the same information, but filtered to only show user and system tables respectively.

Table 27-13. pg_statio_all_indexes View

Column

Type

Description

relid

oid

OID of the table for this index

indexrelid

oid

OID of this index

schemaname

name

Name of the schema this index is in

relname

name

Name of the table for this index

indexrelname

name

Name of this index

idx_blks_read

bigint

Number of disk blocks read from this index

idx_blks_hit

bigint

Number of buffer hits in this index

The pg_statio_all_indexes view will contain one row for each index in the current database, showing statistics about I/O on that specific index. The pg_statio_user_indexes and pg_statio_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.

Table 27-14. pg_statio_all_sequences View

Column

Type

Description

relid

oid

OID of a sequence

schemaname

name

Name of the schema this sequence is in

relname

name

Name of this sequence

blks_read

bigint

Number of disk blocks read from this sequence

blks_hit

bigint

Number of buffer hits in this sequence

The pg_statio_all_sequences view will contain one row for each sequence in the current database, showing statistics about I/O on that specific sequence.

Table 27-15. pg_stat_user_functions View

Column

Type

Description

funcid

oid

OID of a function

schemaname

name

Name of the schema this function is in

funcname

name

Name of this function

calls

bigint

Number of times this function has been called

total_time

double precision

Total time spent in this function and all other functions called by it, in milliseconds

self_time

double precision

Total time spent in this function itself, not including other functions called by it, in milliseconds

The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. The track_functions parameter controls exactly which functions are tracked.

Other ways of looking at the statistics can be set up by writing queries that use the same underlying statistics access functions used by the standard views shown above. For details such as the functions' names, consult the definitions of the standard views. (For example, in psql you could issue \d+ pg_stat_activity.) The access functions for per-database statistics take a database OID as an argument to identify which database to report on. The per-table and per-index functions take a table or index OID. The functions for per-function statistics take a function OID. Note that only tables, indexes, and functions in the current database can be seen with these functions.

Returns a record of information about the backend with the specified PID, or one record for each active backend in the system if NULL is specified. The fields returned are a subset of those in the pg_stat_activity view.

pg_stat_get_snapshot_timestamp()

timestamp with time zone

Returns the timestamp of the current statistics snapshot

pg_stat_clear_snapshot()

void

Discard the current statistics snapshot

pg_stat_reset()

void

Reset all statistics counters for the current database to zero (requires superuser privileges)

pg_stat_reset_shared(text)

void

Reset some cluster-wide statistics counters to zero, depending on the argument (requires superuser privileges). Calling pg_stat_reset_shared('bgwriter') will zero all the counters shown in the pg_stat_bgwriter view. Calling pg_stat_reset_shared('archiver') will zero all the counters shown in the pg_stat_archiver view.

pg_stat_reset_single_table_counters(oid)

void

Reset statistics for a single table or index in the current database to zero (requires superuser privileges)

pg_stat_reset_single_function_counters(oid)

void

Reset statistics for a single function in the current database to zero (requires superuser privileges)

pg_stat_get_activity, the underlying function of the pg_stat_activity view, returns a set of records containing all the available information about each backend process. Sometimes it may be more convenient to obtain just a subset of this information. In such cases, an older set of per-backend statistics access functions can be used; these are shown in Table 27-17. These access functions use a backend ID number, which ranges from one to the number of currently active backends. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. For example, to show the PIDs and current queries of all backends:

SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
pg_stat_get_backend_activity(s.backendid) AS query
FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s;

Table 27-17. Per-Backend Statistics Functions

Function

Return Type

Description

pg_stat_get_backend_idset()

setof integer

Set of currently active backend ID numbers (from 1 to the number of active backends)

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